Submicron silicon photonics waveguides offer the potential of large-scale integration. However, due to the lack of efficient on-chip light sources, there is a need for efficient methods and devices for coupling light from an off-chip optical light source to the silicon photonics circuit.
Given the cost of external laser integration, it may be advantageous to use a small number of such lasers, combined with an on-chip splitter tree distribution network to distribute the optical power generated by a single laser over a plurality of on-chip waveguides. Because in such a configuration each laser needs to feed many parts or photonic sub-circuits of the photonics circuit, the initial optical power coupled to the photonics circuit should be sufficiently high to compensate for a power reduction resulting from branching/splitting and on-chip losses.
In a silicon photonics circuit the amount of optical power that can be efficiently guided in a submicron wire waveguide is limited by nonlinear processes: two-photon absorption and subsequently free carrier absorption. These effects start occurring at an optical power as low as 10 mW in a 450 nm wide silicon strip waveguide. At an optical power of 50 mW the additional propagation loss due to nonlinear processes is about 3 dB/cm. This limits the maximum optical power that can be coupled to an on-chip submicron waveguide from an external light source, and therefore it limits the number of on-chip sub-circuits that can be powered from this single external light source.